Showing papers on "Rarefaction published in 2003"

Effect of Surface Roughness on Nitrogen Flow in a Microchannel Using the Direct Simulation Monte Carlo method

Abstract: The paper presents direct simulation Monte Carlo (DSMC) results for nitrogen flow in a microchannel with surface roughness modeled by an array of rectangular modules placed on one side of a parallel-plate channel. The effects of relative surface roughness, roughness distribution, and gas rarefaction on flow are investigated and the results are presented in the form of the product of friction factor and Reynolds number. It was found that the effect of surface roughness is more pronounced at low Knudsen numbers. At high Knudsen numbers, rarefaction reduces the interaction between the gas molecules and the channel walls and results in a lower friction factor. The roughness distribution represented by the ratio of the roughness height to spacing of the modules has a significant effect on the flow and friction factor. Finally, the locally fully developed (LFD) flow model can be used to predict gas flow in a microchannel with low values of relative surface roughness.

Glass-modified stress waves for adhesion measurement of ultra thin films for device applications

Abstract: Laser-generated stress wave profiles with rarefaction shocks (almost zero post-peak decay times) have been uncovered in different types of glasses and presented in this communication. The rise time of the pulses was found to increase with their amplitude, with values reaching as high as 50 ns . This is in contrast to measurements in other brittle crystalline solids where pulses with rise times of 1 – 2 ns and post-peak decay times of 16 – 20 ns were recorded. The formation of rarefaction shock is attributed to the increased compressibility of glasses with increasing pressures. This was demonstrated using a one-dimensional nonlinear elastic wave propagation model in which the wave speed was taken as a function of particle velocity. The technological importance of these pulses in measuring the tensile strength of very thin film interfaces is demonstrated by using a previously developed laser spallation experiment in which a laser-generated compressive stress pulse in the substrate reflects into a tensile wave from the free surface of the film and pries off its interface at a threshold amplitude. Because of the rarefaction shock, glass-modified waves allow generation of substantially higher interfacial tensile stress amplitudes compared with those with finite post-peak decay profiles. Thus, for the first time, tensile strengths of very strong and ultra thin film interfaces can be measured. Results presented here indicate that interfaces of 185-nm-thick films, and with strengths as high as 2.7 GPa , can be measured. Thus, an important advance has been made that should allow material optimization of ultra thin layer systems that may form the basis of future MEMS-based microelectronic, mechanical and clinical devices.

Double Layers in Expanding Plasmas and Their Relevance to the Auroral Plasma Processes

Abstract: When a dense plasma consisting of a cold and a sufficiently warm electron population expands, a rarefaction shock forms. In the expansion of the polar wind in the magnetosphere, it has been previously shown that when a sufficiently warm electron population also exists, in addition to the usual cold ionospheric one, a discontinuity forms in the electrostatic potential distribution along the magnetic field lines. Despite the lack of spatial resolution and the assumption of quasi-neutrality in the polar wind models, such discontinuities have been called double layers (DLs). Recently similar discontinuities have been invoked to partly explain the auroral acceleration of electrons and ions in the upward current region. By means of one-dimensional Vlasov simulations of expanding plasmas, for the first time we make here the connection between (i) the rarefaction shocks, (ii) the discontinuities in the potential distributions, and (iii) DLs. We show that when plasmas expand from opposite directions into a deep density cavity with a potential drop across it and when the plasma on the high-potential side contains two electron populations, the temporal evolution of the potential and the plasma. distribution generates evolving multiple double layers with an extended density cavity between them. One of the DLs is the rarefaction-shock (RFS) and it forms by the reflections of the cold electrons coming from the high-potential side; it supports a part of the potential drop approximately determined by the hot electron temperature.

A Gas-Solid Free Boundary Problem for a Compressible Viscous Gas

Abstract: In this paper we propose a gas-solid free boundary problem for a one-dimensional model system of a compressible viscous gas associated with the inflow problem, where the solid constantly changes into the gas and produces the inflow of the gas on the free boundary. We first show the existence of the traveling wave solution and its asymptotic stability. We further discuss the case in which the asymptotic state of the solution is given by a combination of the traveling wave solution and rarefaction wave.

On the Stability of Rarefaction Wave Solutions for Viscous p-system with Boundary Effect

Abstract: The inflow problem in the supersonic case for a one-dimensional compressible viscous gas on the half line (0,+∞) is investigated. A stability theorem concerning the long time behavior of rarefaction wave is established.

Numerical Study of Shock Compression of Graphite and Its Conversion to Diamond in Conical Targets

Abstract: The shock compression of carbon placed into lead conical targets that are subjected to a blow from aluminum strikers moving at a velocity of 4 km/s is numerically studied. The statement of the problem includes the hydrodynamic equations for compressed media, semi-empirical wide-range equations of state, and a kinetic model of nonequilibrium graphite-to-diamond conversion that is calibrated with known experimental data. All stages of the process are analyzed, starting from the cumulative effect at the symmetry axis of the target to the release of the sample due to the rarefaction wave coming from the rear side of the striker with allowance for spalling.

Investigation of the throw distance of sputtered atoms under consideration of gas rarefaction

Abstract: The throw distance (TD) of sputtered atoms of a variety of elements was measured via the pressure-dependent atomic deposition rate. For each element the pressure-dependent rate was determined at a low- and a high sputtering power with typical nominal power-densities of 0.3 and 4 W/cm2, respectively. The Keller–Simmons formula, which contains the TD as an explicit fit parameter, well describes the functional dependence of rate on pressure. For low-mass targets (Al, Si, V), the TD was found to be independent of sputtering power over the range investigated. At increased atomic mass—first observed for Cr—increasing sputtering power causes an increase of the TD, what is related to the stronger gas rarefaction for these elements. It is estimated that a gas density reduction by less than 20% is uncritical for an accurate determination of the TD. The TDs of the elements (measured at negligible gas rarefaction) are found to depend linearly on the product of velocity persistence in argon and surface binding energy. The velocity persistence is fairly proportional to the logarithm of the atomic mass for elements heavier than 20 amu.

Linear perturbation growth at the trailing edge of a rarefaction wave

Abstract: An analytic model for the perturbation growth inside a rarefaction wave is presented. The objective of the work is to calculate the growth of the perturbations at the trailing edge of a simple expanding wave in planar geometry. Previous numerical and analytical works have shown that the ripples at the rarefaction tail exhibit linear growth asymptotically in time [Yang et al., Phys. Fluids 6, 1856 (1994), A. Velikovich and L. Phillips, ibid. 8, 1107 (1996)]. However, closed expressions for the asymptotic value of the perturbed velocity of the trailing edge have not been reported before, except for very weak rarefactions. Explicit analytic solutions for the perturbations growing at the rarefaction trailing edge as a function of time and also for the asymptotic perturbed velocity are given, for fluids with γ<3. The limits of weak and strong rarefactions are considered and the corresponding scaling laws are given. A semi-qualitative discussion of the late time linear growth at the trailing edge ripple is presented and it is seen that the lateral mass flow induced by the sound wave fluctuations is solely responsible for that behavior. Only the rarefactions generated after the interaction of a shock wave with a contact discontinuity are considered.

Convergence to strong nonlinear rarefaction waves for global smooth solutions of $p-$system with relaxation

Abstract: This paper is concerned with the large time behavior of global smooth solutions to the Cauchy problem of the p-system with relaxation. Former results in this direction indicate that such a problem possesses a global smooth solution provided that the first derivative of the solutions with respect to the space variable x are sufficiently small. Under the same small assumption on the global smooth solution, we show that it converges to the corresponding nonlinear rarefaction wave and in our analysis, we do not ask the rarefaction wave to be weak and the initial error can also be chosen arbitrarily large.

On the Stability of Rarefaction Wave Solutions for Viscous p—system with Boundary Effect

Abstract: The inflow problem in the supersonic case for a one-dimensional compressible viscous gas on the half line (0, +∞) is investigated. A stability theorem concerning the long time behaviour of rarefaction wave is established.

Two-Phase Models of Formation of Cavitating Spalls in a Liquid

Abstract: The dynamics of the structure of a liquid layer structure (with microbubbles of a free gas) behind a rarefaction wave front is studied numerically using the two-phase Iordansky–Kogarko–van Wijngaarden model and the “frozen” mass-velocity field model. An analysis of the initial stage of cavitation by the Iordansky–Kogarko–van Wijngaarden model showed that tensile stresses behind the rarefaction wave front relax quickly and the mass-velocity field in the cavitation zone turns out to be “frozen.” This effect is used to describe the late stage of the development of the cavitation zone. These models were combined to study the formation of cavitating spalls in a free-surface liquid under shock-wave loading.

Flow separation between rotating eccentric cylinders

Abstract: We deal with the dynamic and thermal characteristics of the rarefied flow between two eccentric isothermal cylinders one of which is rotating. A numerical program of direct simulation which is based on a Monte Carlo (DSMC) procedure was adopted and the main results concern the influence of the Knudsen and Mach numbers, and of the eccentricity on the location and extent of the flow separation. Part of the flow domain is shown to be cooler than the wall temperature, and the extent of this region decreases with an increasing gas rarefaction.

On 2x2 conservation laws with large data

Abstract: We deal with a strictly hyperbolic system of two conservation laws in one spatial dimension One of the eigenvalues of the system is of Temple type (rarefaction and shock curves coincide), the other eigenvalue is only required to be genuinely nonlinearWe consider the initial value problem for data of the following kind: the total variation of the Temple component is bounded, possibly large, while the total variation of the other component is small For such data we prove global existence, uniqueness and L¹-Lipschitz continuous dependence of solutions

Method of performing homogeneous and heterogeneous reactions by means of plasma

Abstract: FIELD: chemical technologies; electronics; chemical industry; metallurgy. SUBSTANCE: proposed method includes delivery of reactive gas from reactive gas source to vacuum reaction chamber where supersonic flow is formed in such way that rarefaction zone is formed at chamber inlet in its center portion at reduced density relative to adjacent parts of flow; said flow of reactive gas is subjected to activation by acting on it with electron beam introduced in rarefaction zone. EFFECT: enhanced productivity; reduced capital outlays; increased utilization factor. 5 cl, 4 dwg

Self-Action of Wave Beams Containing Shock Fronts

Abstract: We briefly review the effects of nonlinear self-action of beams of strongly distorted waves containing steep shock fronts. The features of inertial self-actions of periodic sawtooth waves in quadratic nonlinear media without dispersion are discussed. These phenomena can be caused by an acoustic wind or thermal lens formed as a result of the nonlinear dissipation at the shock fronts. Instantaneous self-actions are analyzed on the examples of periodic trapezoidal waves, which are formed in cubic nonlinear media and contain alternating compression and rarefaction shocks, and a single-pulse signal containing a shock front. Mathematical models and solutions to the corresponding nonlinear equations are given. A qualitative comparison with optical self-action phenomena and with available experimental data is performed.

A Shock-Wave Generator of a Single Cavitation Bubble

Abstract: A modified version of a flat electromagnetic converter for enhancing the amplitude of the pressure of a shock wave using the Laptev-Trishin method and reducing the duration of an acoustic pulse is described. The interference of a focused edge diffraction rarefaction wave and a wave reflected from a free surface is used to form a narrow space-time negative-pressure region. The conditions for generating a single cavitation bubble near the free surface are found.

Method and device for treating pains

Abstract: FIELD: medicine; medical engineering. SUBSTANCE: method involves applying passive local muscle extension in the zones containing trigger points. The tension is created by means of rarefaction sucking disks having perforated elastoplastic membranes applied to skin areas under which muscle lesion zones are available. The treatment is carried out in pulsating mode with pulse repetition frequency of 0.1-1.0 Hz when varying rarefaction in sucking disk from 5 to 78 kPa. Device has vacuum sucking disk designed as hollow rigid bell-shaped casing having perforated elastoplastic membrane fixed along its perimeter possessing directed change of elastoplastic properties from one edge to the other and rarefaction source. EFFECT: enhanced effectiveness in creating integral composite of membrane-skin-subcutaneous fat-muscle and acting upon muscle in given direction. 9 cl, 4 dwg

An integral model for thermal backscattering from the exhaust plume of a space-based HF laser

Abstract: The operation of a space-based HF laser may be hampered due to self contamination by corrosive exhaust products. We estimate one effect contributing to contaminating backflow: thermal backscattering from the rarefaction fans flanking the exhaust ring-jet. Our computational model is based on a first-iterate approximation to the Boltzmann equation in integral form. Results indicate that thermal backscattering of corrosive speies (HF, DF) is negligible.

Nonideal plasma under extreme conditions generated by shock waves

Abstract: Summary form only given, as follows. The physical properties of strongly coupled matter at high energy densities are analyzed in a broad region of the phase diagram. The theoretical and experimental methods of hot dense matter investigations are discussed. The main attention is paid to shock wave methods. Intense shock, rarefaction and radiative waves in gaseous, solid and porous samples, explosion and bulk electron and ion heating were used for generation of extremely high temperatures and high pressures. The highly time-resolved diagnostics allow us to measure the thermodynamical, radiative and mechanical properties of high temperature condensed matter in the broad region of the phase diagram from compressed condensed solid states up to the low density gas range, including high temperature evaporation curves with near-critical states of metals, strongly coupled plasma, and metal-insulator transition regions. Thermodynamical parameters of metal critical points are analyzed and compared with the theoretical predictions. The shock-wave-induced nonequilibrium phenomena at fast melting and adiabatical condensation are analyzed in the framework of the interspinodal decomposition model. The theoretical interpretation of the opacity measurements demonstrates strong deformation of discrete spectrum in coupled materials. The pressure ionization phenomena in hydrogen, iodine, silica, sulfur, fullerenes, and some metals are analyzed on the base of multiple shock compression experiments. The spall strength of single and polycrystal metals at extremely high deformation rates produced by fast shock waves is discussed.

Study of interaction between shock wave and unsteady boundary layer

Abstract: This paper reports theoretical and experimental study of a new type of interaction of a moving shock wave with an unsteady boundary layer. This type of shock wave-boundary layer interaction describes a moving shock wave interaction with an unsteady boundary layer induced by another shock wave and a rarefaction wave. So it is different from the interaction of a stationary shock wave with steady boundary layer, also different from the interaction of a reflected moving shock wave at the end of a shock tube with unsteady boundary layer induced by an incident shock. Geometrical shock dynamics is used for the theoretical analysis of the shock wave-unsteady boundary layer interaction, and a double-driver shock tube with a rarefaction wave bursting diaphragm is used for the experimental investigation in this work.